The present invention relates to the field of fugitive dyes, and more particularly to the class of fugitive dyes that are prepared by attaching a dyestuff compound or ion to a water-soluble polymeric compound.
Dyes that are capable of imparting an intense yet removable color to a substrate are useful in a number of applications, including fugitive tinting of fabrics in textile manufacturing and temporary markers for arts and crafts. Dyes that are washable, in the sense of being removable from the substrate by scrubbing with aqueous soap or detergent, have been known in the art for nearly 50 years. The patent to Kuhn (U.S. Pat. No. 3,157,633), issued in 1964, taught a new class of chromophoric surfactants, in which one or more water-soluble polymeric groups comprising multiple ethyleneoxy units are coupled to a dyestuff moiety. In the Kuhn dye formulations, the surface active properties of the polymeric groups are used to counteract the affinity of the dyestuff radical for the substrate in order to produce a washable dye.
The Kuhn patent also discloses that the fugitivity of these polyethyleneoxy tints is improved when the dyestuff compound contains at least one sulfonic acid group (SO3−) to increase solubility. The patent to Brendle (U.S. Pat. No. 4,167,510) similarly teaches that the presence of strongly acidic (pKa<3) ionic groups (e.g. sulfonates or phosphates) in the dyestuff compound increases water-solubility, and hence fugitivity. While both Kuhn and Brendle aim to enhance fugitivity by making the dyestuff compound more soluble, however, the presence of a strongly acidic solubilizing group, such as sulfonic acid, in the dyestuff moiety actually increases its affinity for the substrate (its “substantivity”), thereby limiting the fugitivity of the dye.
Dye molecules (“chromogens”) are aromatic compounds comprising one or more aryl rings with delocalized p-orbital electron systems. The delocalized electron system of a given aryl ring will maximally absorb electromagnetic radiation at a characteristic wavelength λmax. For example, an isolated benzene ring will maximally absorb electromagnetic radiation at a wavelength λmax of 254 nm, which is in the ultraviolet (UV) portion of the electromagnetic spectrum. Therefore, liquid benzene appears colorless, because the human eye does not detect UV radiation. On the other hand, the alkaline form of the acid-base indicator methyl orange, which is an azo-benzene comprising two phenyl rings linked by an N═N bond, has a λmax of 440 nm, which falls within the visible range of the electromagnetic spectrum (390-750 nm). When 440 nm wavelength light (blue) is removed from the visible spectrum, the complementary color (yellow) is observed, and thus alkaline methyl orange appears yellow.
Benzene and azo-benzene are examples of organic compounds having conjugated systems of alternating single and double covalent bonds. The more extended the conjugated system is, i.e., the greater the number of alternating single and double bonds, the longer will be the peak electromagnetic absorption wavelength λmax of the compound. This is because the extent of π-electron delocalization increases with the extent of conjugation, because conjugation produces an overlapping system of p-orbitals. With increasing delocalization, the energy difference between the π (bonding) and π* (anti-bonding) orbitals is reduced. Since the energy E of a photon absorbed by a π electron in jumping to a π* orbital is related to the photon's wavelength by the expression E=hc/λ (where h is Planck's constant and c is the speed of light), increasing π electron delocalization results in electromagnetic absorption at longer peak wavelengths. Consequently, extension of the conjugated system tends to shift the peak absorption wavelength λmax toward the red end of the visible spectrum, which causes a complementary shift toward the violet end of the spectrum in the observed color associated with a dye compound.
In addition to extension of conjugated systems of single/double bonds, electron delocalization in a dye compound can also be increased by the presence in the molecule of functional groups or substituents having lone pairs of electrons, since these non-bonding n-electrons can also be promoted to a π* orbital at the lower transition energy levels associated with electromagnetic wavelengths in the visible range. Lone pair electrons often occur in O, N or S atoms in groups such as —OH, —NH2, —NHR, —NR2, and —SH (where R is an alkyl or aryl group).
The chromophore is the portion of a dye molecule that contains its color-producing moiety, which has delocalized π electrons shared among conjugated aromatic rings. Important chromophoric functional groups are —N═N-(azo), —C═C— (ethenyl), —C═O— (carbonyl), —C═N— (amino), —O═N—O— (nitro). The intensity of the color depends on the number of such chromophoric groups. Benzene, for example, is colorless because it lacks a chromophore, but the presence of the chromophoric nitro group in nitrobenzene imparts a pale yellow color to the molecule.
Auxochromes are functional groups with non-bonded electrons that have the effect of deepening and intensifying the color produced by the chromophores to which they are attached. Going back to our example of pale yellow nitrobenzene, if we attach an hydroxyl (—OH) radical to the chromophore, the hydroxyl group functions as an auxochrome, and the resulting compound, p-hydroxynitrobenzene, exhibits a deep yellow color. Moreover, if an auxochrome with lone-pair electrons is in direct conjugation with the delocalized π-electron system of the chromophore, it can extend the conjugated system and increase the peak absorption wavelength λmax of the chromophore, thereby altering or adjusting the color.
While the presence of chromophores in a conjugated aromatic system is sufficient to provide color, it is not sufficient to constitute a dye. A dye, in addition to having color, must also have the ability to penetrate and bond to a substrate. To do this, the dye molecule must contain functional groups that can ionize so as to provide solubility and substantivity. These functional groups, which are attached to the chromophore, are called solubilizing groups. Solubilizing groups can either be acidic, such as the sulfonic radical —HSO3, or a neutral salt, such as sodium sulfonate NaSO3−.
By controlling both the solubility and substantivity of the chromogen, solubilizing groups hold the key to optimizing the fugitivity of a dye. Heretofore, the prior art fugitive dye formulations have exploited only the solubilizing function in selecting solubilizing groups. By neglecting substantivity, however, the existing formulations have limited the degree of achievable fugitivity to so-called “washability”, which means that the substrate has to be subjected to scrubbing or agitation in the presence of soap or detergent in order to remove the dye color. On the other hand, dyes formulated in accordance with the present invention exhibit “rinsability”, which means that the dye color can be removed by simply rinsing the substrate in water or under running water.
Rinsability, as opposed to washability, has several practical advantages. For example, fugitive dyes incorporated in children's art supplies, such as markers and finger paints, will stain skin and clothing, as well as furniture and carpeting. The ability to rinse away such stains under a faucet or hose provides a level of convenience not afforded by washable dyes. Similarly, in commercial applications, such as fugitive tint textile marking, rinsable dyes can shorten process times and cut production costs significantly.